In general, in a solid-state image pickup device having a discrete pixel structure, or an image pickup optical system having an image tube, in turn, having a color separation filter, an image of an object is processed with optical spatial sampling for generating an image output.
The degree of fineness of a picture pattern that can be handled by such image pickup optical system is determined in connection with the sampling frequency and, if spatial frequency components higher than the sampling frequency are contained in the picture pattern, spurious signals known as moire or aliasing tend to be produced. For this reason, it becomes necessary to arrange an optical low-pass filter for limiting the high spatial frequency components of the object in the image pickup optical system.
As such optical low-pass filter, an optical low-pass filter taking advantage of double refraction of a quartz plate has been employed. The optical low-pass filter by the quartz plate affords optical blur by double refraction proper to the quartz plate for reducing high spatial frequency components of the object image for suppressing spurious signals.
As an example of the optical low-pass filter employing such quartz plate, there is known an optical low-pass filter employing three quartz plates, as proposed by JP Patent Kokai Publication No. 57-39683 (1982).
The MTF--spatial frequency characteristics of an optical low-pass filter employing three quartz plates and an optical low-pass filter employing a sole low-pass filter are explained in connection with FIGS. 1 and 2. The optical low-pass filter employing three quartz plates are low in MTF values for a frequency not less than the cut-off frequency of 80 lp/mm, as shown in FIG. 1, such that it has sufficient spurious signal inhibiting effects. Conversely, the optical low-pass filter employing a sole low-pass filter has larger MTF values for a frequency not less than the cut-off frequency of 80 lp/mm, as shown in FIG. 2, such that it has only poor effects in inhibiting spurious signals.
Consequently, the conventional practice has been to arrange the optical low-pass filter with three or more quartz plates to suppress the MTF for the frequency higher than the cut-off frequency.
However, since the optical low-pass filter in the conventional image pickup optical system is constituted by three or more quartz plates, the following problems arise.
That is, quartz crystal growth in general involves a lot of time and hence quartz is rather costly. Besides, since as many as three or more of the costly quartz plates are used, the image pickup optical system and hence the image pickup apparatus employing the image pickup optical system becomes expensive. In addition, since three or more quartz plates are bonded together to fabricate the optical low-pass filter, the operation of positioning the quartz plates being bonded together becomes troublesome to give rise to a complicated manufacture process and an increased number of process steps.